Injection drug use (IDU) remains the dominant mode of HIV and hepatitis C virus transmission in China. Accumulating evidence demonstrates that risk for HIV and HCV infection is highest among young injection drug users who have recently initiated drug injection. A key challenge in HIV/HCV prevention is therefore to prevent the transition from non-injection drug use (NIDU) to IDU through interventions. However, social or personal factors important for the transition remain poorly understood in China. The objective of this application is to examine the roles of social network components as they contribute to the transition from NIDU to IDU among young drug users between the ages of 18 and 25 years. Social networks are conceptualized as comprising three components: relations (kin, friend, coworkers, drug users, and sex partner);structures (size, density, concurrency and multiplexity) and functions (social support and norms). Network members exert powerful influences on normative determinants of intention and behaviors. The use of a social network approach for studying the transition may be particularly useful in the Chinese culture which emphasizes collectivism. Collectivism focuses on a social network that is composed of people who consider themselves to be closely linked with one another. The central hypothesis of the application is that network functions, especially social norms, mediate the associations between the other two network components (relations and structures) and IDU transition in the Chinese collectivist culture. This hypothesis will be tested in a sample drawn from young drug users'social network ties, using respondent-driven sampling (RDS). This study has two aims: (1) to develop a contextualized understanding of the linkages among demographic characteristics, the Chinese collectivist culture, network components and the transition to IDU through qualitative research;and (2) to examine and test the central hypothesis through a quantitative approach. In-depth interviews will be performed among 24 young drug users. A cross-sectional survey will be conducted among 250 young drug users (including both IDUs and NIDUs) recruited through RDS from drug users'network ties. Through the use of both qualitative and quantitative research approaches, we will explore the Chinese culture, social norms and values in order to accomplish the listed aims. The results of this current study and experience gained from it will facilitate the formulation of a network-based intervention narrative. This project seeks to determine the roles of social network components as they contribute to the transition from non-injection drug use to injection drug use among young drug users in China. The results of this study and experience gained from it will facilitate the formulation of a network-based intervention.